1. Field of the Invention
The present invention relates to a sheet conveying apparatus for conveying sheets and to an image forming apparatus.
2. Description of the Related Art
Conventionally, an apparatus body of an image forming apparatus selected from the group consisting of a photocopier, a printer and the like for forming an image onto a sheet occasionally conveys a sheet where an image is formed with an apparatus body and includes a sheet stacking apparatus (stacker) for stacking a large amount of the conveyed sheets. Such a stacker is described in Japanese Patent Application Laid-Open No. 2006-124051. A stacker 100 in FIG. 8 is described in Japanese Patent Application Laid-Open No. 2006-124051.
A sheet discharged from the apparatus body of an image forming apparatus is conveyed to an inlet roller pair 101 and is guided upward by a flapper 102. A sheet is further guided upward and is discharged into a sample tray 109 above a stacker. Even if the sheet is guided upward, in the case where the sheet is conveyed in a normal state, the sheet passes a conveying path 103 to be conveyed further from outlet roller pair 104 to a post processing apparatus (not illustrated in the drawing) in the downstream. In addition, in the case where the sheet is guided to a downward path by the flapper 102, the sheet is discharged and stacked on a stacking tray 106 by a discharging roller 105. The stacking tray 106 moves up and down with an elevating apparatus not illustrated in the drawing and is adjusted in height so that the height on the upper plane of the stacking sheet is always kept constant with a sheet plane detecting lever 110.
However, in that stacker, the stacking tray 106 is horizontal. Therefore, in the case of a sheet such as an easily bending thin sheet, the sheet is discharged from the outlet roller pair 104 so that the tip of the sheet droops and rubs against already stacked sheets to occasionally result in jamming.
Therefore, the stacker 100 illustrated in FIG. 8 conveys the tip of the sheet discharged from a discharging roller 105 being gripped by grippers 107a and 107b on a belt 108. The tip of the sheet is urged to the stopper 114 and the sheet is disengaged from the grippers 107a and 107b. Thereby, the sheet drops onto the stacking tray 106 and is stacked. The belt 108 is hung by a drive pulley 111 and a driven pulley 112 to circulate in the direction of an arrow A. The grippers 107a and 107b are provided with a common interval on the belt 108.
However, in the stacker 100 adapted as described above, if grippers are arranged so as to be capable of conveying and stacking sheets with half sheet sizes A4, B5 and the like at an occasion of conveying a sheet with large sheet sizes A3, B4 and the like, there are present grippers 107a and 107b which do not grip the sheet. The grippers which do not grip a sheet will hamper conveyance of a sheet to occasionally damage an image while an image is formed on the sheet.
Therefore, in a sheet conveying apparatus in conventional stacker, grippers are arranged in an interval to allow a sheet of the maximum length to be stacked to be conveyed. However, when the grippers are thus arranged, the distance for conveying a sheet gets long to make sheet conveying efficiency worse when a short sheet in length in the sheet conveying direction is conveyed. In that case, when a short sheet in length in the sheet conveying direction is conveyed, acceleration of the circulation velocity of the belt is conceivable. However, a change in the circulation velocity of the belt based on sheet length gives rise to another problem that structure and controls get complicated.
In addition, in an apparatus disclosed in Japanese Patent Application Laid-Open No. S61-174058, a pair of drive shafts provided at a predetermined spaces and four sprockets 913 being present in each of those drive shafts are axially arranged in a position so as to face each other as illustrated in FIG. 9A and FIG. 9B. The four sprockets 913 are divided into two outer sprockets 913 arranged in both ends of the drive shaft and two units are arranged inside the outer sprockets 913. Four roller chains 914 are respectively attached to those sprockets 913 in mesh with each other and rotate corresponding with rotation of respective sprockets 913. The roller chains 914 consist of two chains put on the inner sprockets 913 respectively and other two chains put on the outer sprockets 913 respectively. The two chains arranged inside and two chains arranged outside come into communication with a plurality of tie bars 915 and 916 mounted on chains 914 respectively to form pair structure. The distance between two roller chains 914 brought into communication with the tie bars 915 and 916 substantially corresponds with the width of a sheet (length in the direction perpendicular to the conveying direction) in the sheet size A3. Each sprocket 913 on the side of the respective drive shaft is axially connected to those two sets of roller chains 914 through a clutch 917 and can, therefore, rotate independently. Relative positions of a plurality of tie bars 915 fixed on the roller chains 914 and a diaper 916 is freely variable in the direction of movements of the roller chains 914. Tie bars 915 and 916 are provided on the outer surface thereof with a gripper 918 capable of gripping the tip of the sheet.
In the case where that apparatus conveys a sheet 919 with a sheet size A3, as illustrated in FIG. 9A, two sets of roller chains 914 are provided so that they are changed in their relative positions by the clutch 917 to move the gripers 918 of both chains so as to come substantially closer to each other. The tip of the sheet 919 with a sheet size A3 is gripped by grippers 918 (grippers provided in the outer roller chain in FIG. 9A) located on the delayed side in the sheet moving direction and is conveyed. At that occasion, the grippers of the inner roller chain do not hold a sheet and enter a rotary or non-rotary state in cooperation with outside roller chain.
In the case of conveying a sheet 920 with the sheet size A4, the relative positions of the two sets of roller chains 914 are changed as illustrated in FIG. 9B. That is, the positions of the tie bars 915 and 916 are moved so that the tie bars 915 of the inner roller chain 914 come in substantially center position of two tie bars 916 and 916 provided in the outer roller chain 914. The distance between the mutually adjacent tie bars 915 and 916 is set to the length of the sheet with the sheet size A4. Thus, once the relative positions of the two sets of roller chains 914 are set subjected to displacement in the longitudinal direction of the roller chain, then both of the roller chains 914 and 914 start rotating in cooperation with each other to convey the sheet 920 with the gripper 918 of both of the roller chains 914 and 914.
In the case of conveying a sheet 920 with the sheet size A4, the relative positions of the two sets of the roller chains 914 are changed as illustrated in FIG. 9B. That is, the positional relation of the tie bars 915 and 916 is adjusted so that the tie bar 915 of the inner roller chain 914 is arranged in the substantially center position of the two tie bars 916 provided in the outer roller chain 914. The distance between the mutually adjacent tie bars 915 and 916 is set to the sheet length of a sheet with the sheet size A4. Thus, when the relative positions of the two sets of roller chains 914 are set, then both of the roller chains 914 start rotating in cooperation with each other to convey the sheet 920 with the grippers 918 of both of roller chains 914.
In an apparatus disclosed in that Japanese Patent Application Laid-Open No. S61-174058, a gripper holding no sheet at an occasion of conveying a sheet with a sheet size A3 is located in the position in the vicinity of the gripper 918 which conveys the sheet. That is, at an occasion of conveying a sheet with the sheet size A3, the gripper 918 conveying the sheet and the gripper conveying no sheet will be in an aligned state in the sheet conveying direction. Here, in order to prevent the grippers holding no sheet refrains from contacting the sheet to be conveyed to damage the sheet, the space for the grippers holding no sheet has to be secured in the direction of conveying the sheet. Accordingly, the distance among a plurality of grippers gripping a sheet has to be lengthened for the space secured for the grippers regaining no sheet. Accordingly, it is necessary to lengthen the distance between the grippers for conveying the sheet and, therefore, sheet conveyance efficiency is not good.